covariant type in java

class A {} 
class B extends A {} 
class Base { 
    A fun() { 
        System.out.println("Base fun()"); 
        return new A(); 
    } 
} 
class Derived extends Base { 
    B fun() { 
        System.out.println("Derived fun()"); 
        return new B(); 
    } 
}
public class Main { 
    public static void main(String args[]) { 
       Base base = new Base(); 
       base.fun(); 
       Derived derived = new Derived(); 
       derived.fun(); 
    } 
}